Known separating apparatus include those used in vacuum cleaners, for example cyclonic separating apparatus. Such cyclonic separating apparatus are known to comprise a low efficiency cyclone for separating relatively large particles and a high efficiency cyclone located downstream of the low efficiency cyclone for separating the fine particles which remain entrained within the airflow (see, for example, EP 0 042 723B).
Irrespective of the type of separating apparatus used, there may be a risk of a small amount of dirt and dust passing through the separating apparatus and being carried to the motor-driven fan unit. It is undesirable for dirt and dust particles to pass through the fan of a motor and fan unit because the fan may become damaged or may operate less efficiently.
In order to reduce this problem, some vacuum cleaners include a fine filter in an air flow path between the separating apparatus and the airflow generator. This filter is commonly known as a pre-motor filter and is used to extract any fine dirt and dust particles remaining in the air flow after it has passed through the separating apparatus.
It is also known to provide a filter in an air flow path downstream of the air flow generator in order to extract any remaining dirt and dust particles prior to the air flow exiting the appliance. This type of filter is known as a post-motor filter. The post-motor filter also captures particles produced by the brushes of the motor.
Filter assembly are used on the Dyson range of vacuum cleaners, for example, on model numbers DC04, DC07, DC12, DC14 and DC15. The principle by which filter assemblies of this type operate is described in GB 2349105 and EP 1239760B.
In vacuum cleaner applications it is desirable for the dust separating efficiency to be as high as possible while maintaining suitable filter lifetime.